用递归算法判断你是否为回文

package firs;

import java.util.Scanner;

public class Digui {
private  static int a;
private static int b=0;
public static void ok(String str,int i)
{
    if(i==a/2||i==a/2+1)
    {
        System.out.println("yes");
    }
    else 
    {
        if(str.charAt(i-1)==str.charAt(b))
        {
            i--;
            b++;
            ok(str,i);
        }
        else 
        {
            System.out.println("no");
        }
    }
}
public static void main(String[] args) {
    Scanner input=new Scanner(System.in);
    String str;
    str=input.next();
    a=str.length();
    ok(str,a);
}
}